Vassal stores the game information in a file called swminis.vmod, you'll need to edit that file.
Open up vassal and select "edit module" from the file menu.
Find swminis.vmod wherever it is on your computer.
The game properties are organized into folders, you want to open the one called "Maps"
Right click on "Map Boards" and select "Add Board"
You'll be prompted to name the map and asked for a file location for the image.
Once you've done that you'll need to add a grid to the image to show where the squares are.
Right click on your new map board and select "Add rectangular grid"
You want to set all the numerical values to 50
Check the boxes for "show grid", "edges are legal locations", and "corners are legal locations" and pick a color for the grid.
Hit save and you're done.
You'll end up with something that looks like this:
